Assessing baroreflex gain from spontaneous variability in conscious dogs: role of causality and respiration

A double exogenous autoregressive (XXAR) causal parametric model was used t o estimate the baroreflex gain (alpha (XXAR)) from spontaneous R-R interval and systolic arterial pressure (SAP) variabilities in conscious dogs. This model takes into account 1) effects of current and past SAP variations on the R-R interval (i.e., baroreflex-mediated influences), 2) specific pertur bations affecting R-R interval independently of baroreflex circuit (e.g., r hythmic neural inputs modulating R-R interval independently of SAP at frequ encies slower than respiration), and 3) influences of respiration-related s ources acting independently of baroreflex pathway (e.g., rhythmic neural in puts modulating R-R interval independently of SAP at respiratory rate, incl uding the effect of stimulation of low-pressure receptors). Under control c onditions, alpha (XXAR) = 14.7 +/- 7.2 ms/mmHg. It decreases after nitrogly cerine infusion and coronary artery occlusion, even though the decrease is significant only after nitroglycerine, and it is completely abolished by to tal arterial baroreceptor denervation. Moreover, alpha (XXAR) is comparable to or significantly smaller than (depending on the experimental condition) the baroreflex gains derived from sequence, power spectrum [at low frequen cy (LF) and high frequency (HF)], and cross-spectrum (at LF and HF) analyse s and from less complex causal parametric models, thus demonstrating that s impler estimates may be biased by the contemporaneous presence of regulator y mechanisms other than baroreflex mechanisms.